1. Field of the Invention
The present invention relates to a membrane apparatus for treatment of compressed air and production of dried air or, alternatively, modified nitrogen-rich air, which are free from impurities and have a high degree of drying, and in particular to a spray-painting apparatus using treated compressed air as a carrier fluid in spray-painting.
The invention offers the possibility of varying the exit temperature on use, in particular of the compressed dry air or modified nitrogen-rich air used as a carrier fluid in spray-painting devices.
2. Prior Art
It is known that spray-painting devices of known type consist of guns or nozzles, into which a flow of air under pressure (about 3-5 bars) and a supply of the paint to be sprayed are conveyed.
The air acts as a vector, entraining the paint therewith and transferring it to the surface to be painted.
Applicative examples of spray painting by compressed air are car bodies, furniture, articles made of plastic materials reinforced by incorporated fibreglass, boats, airplanes, leather articles in general, etc.
It is also known that in order to improve the painting quality, elimination of every impurity that may bind to the paint is required as well as making drying of the laid-down paint film as more as possible uniform and quick.
In this way, the possibilities that paint may absorb impurities and moisture from the compressed carrier air or even from ambient air are reduced, in particular if working takes place in an uncontrolled environment.
Therefore, in order to improve quality of painting, the compressed-air flow is dried to remove the condensate (making the air pass through a low-temperature refrigeration-cycle drier for example, generally until a dew-point value not lower than +3xc2x0 C.) and filtered to eliminate traces of oil or other impurities (by filtering the air by means of coalescence filters or activated-carbon filters, for example).
In the spray-painting technique referred to as HVLP (high volumes, low pressure of the compressed air) also known is use of a flow of heated air for the purpose of accelerating paint drying and improve transfer efficiency, i.e. exploitation of the paint employed.
Systems of known type however have some drawbacks due to the fact that different paints and surfaces may require different spray conditions (in particular in terms of pressure and temperature of the carrier fluid).
It is the same as regards environmental conditions, since for instance work parameters may change to a great extent between a working carried out to a room temperature of +30xc2x0 C. rather than of +5xc2x0 C.
In addition, known systems are not always satisfactory when high purity of the compressed air and a high drying degree are required.
At all events, the painting systems of known type have the drawback that they are limited to use of air as the carrier fluid, and therefore involve a problem of possible oxidation of the paints.
Accordingly, it is a first aim of the present invention to propose a spray-painting apparatus using treated air, adapted to eliminate the above mentioned drawbacks.
It is another aim to propose an apparatus and method for spray-painting devices, which have a high yielding and are quite reliable in terms of characteristics of the dry compressed air or modified nitrogen-rich air, treated to the different temperatures.
It is a further aim to propose a spray painting apparatus of versatile use and ready adaptation in which hollow-fibre membranes are used to treat the compressed air by drying it, or hollow-fibre membranes to obtain modified air rich in nitrogen are used alternatively or in an interchangeable manner, in both cases being it possible to operate under different pressure and temperature conditions.
Advantageously, a carrier fluid of nitrogen-rich air in a concentration of nitrogen from 95 to 99.9% can be used for spray painting according to the invention.
According to the invention, the temperature of the nitrogen-rich treated air can be adjusted within the range of 5xc2x0 to 60xc2x0 C., obtaining a dew point which can vary from xe2x88x9240xc2x0 C. and xe2x88x9280xc2x0 C.
In this way, the treated air does not have oil impurities, humidity or other impurities which could be dangerous for the quality of painting.
Furthermore, the use of an anhydrous carrier fluid, which has high purity and adjustable temperature irrespective of the room temperature, allows a quick reticulation process of bi-component paints and a quick evaporation of volatile components.
A further aim of the present invention is to propose a method of spray-painting capable of eliminating the drawbacks of known systems.
The foregoing and still further aims are achieved by an apparatus for spray-painting comprising an inlet for air under pressure (AIR IN); a heating unit (G1) to heat the compressed air; an air-drying and/or separation unit (G2) operating by means of hollow-fibre membranes; means (3, 15, 16) for adjusting heating of the treated air; means (13) for adjusting pressure of the treated air; means (10, 19) for regulation of the dew point should the unit (G2) comprise hollow-fibre membranes for air drying, and/or for regulation of the residual percentage of oxygen in the treated air should the unit (G2) comprise hollow-fibre membranes for production of modified air rich in nitrogen; an outlet for the treated air (AIR OUT) to be used as paint carrier fluid.
Also disclosed is a spray-painting method comprising the steps of feeding air under pressure; adjustably heating the air to be treated; treating the heated air by separation membranes to a preestablished and adjustable drying/separation degree; supplying the treated air to an adjusted pressure and temperature as carrier fluid for the paint.